It is known that ultra low melt viscosity polymers are useful for the production of a variety of products such as adhesives, sealants, coatings, non-woven fabrics by melt blown fiber processes, injection-molded components made at a high rate, etc. An ultra low melt viscosity polymer has a melt viscosity of about 300,000 centipoise (hereinafter "cps") or lower. The melt viscosity of an ultra low melt viscosity polymer can be as low as 500 cps or smaller.
However, it is difficult to use a polymerization process to obtain directly polymers of a very low melt viscosity because, due to their particular nature, such polymers can require complex and costly operations during their preparation primarily in relation to the use of solvents, especially operations of separating the polymers from the solvents in which they are prepared. Thus, it has been proposed to prepare olefinic polymers with a relatively high melt viscosity according to the usual polymerization processes and then to subject these polymers to a thermomechanical degradation treatment in the presence of a free radical generator, under such conditions that the melt viscosity of these polymers decreases to the desired value. In theory, during this treatment, the thermal decomposition of the free radical generator, such as a peroxide, can cause the macromolecular chains of the olefinic polymer to break and thus the melt viscosity of the polymer to decrease.
As used herein, a high melt viscosity polymer is a polymer having a melt viscosity 1,000,000 cps or more; and an ultra low melt viscosity polymer is a polymer having a melt viscosity of about 300,000 cps or lower. A polymer with a melt viscosity of about 300,000 cps will have a melt index of approximately 100 dg/min, and is generally regarded as an ultra high melt flow rate polymer with an ultra high melt index. As used herein, the melt viscosity is measured by Brookfield Viscometer using ASTM D2556 at 350 degrees Farenheit (.degree.F.), unless otherwise specified e.g. as measured at 275.degree. F. As used herein, the melt flow rates or melt indices are measured by ASTM 1238 condition E at 190.degree. C. and 2.16 g wt.
The pelletization of thermoplastic materials is of considerable importance for many applications. Pellets, unlike ingots or bars, readily flow in measuring and dispensing apparatuses and the size of pellet charges can be readily controlled to small tolerances. Moreover, unlike powders, they do not form dust and are not ingested by persons working with them. Thus, they provide a highly convenient form for the packaging, storage and use of many thermoplastic polymers.
It is known to carry out the thermomechanical degradation treatment in the presence of a free radical generator in an extruder, either during pelletization of the polymer or during the conversion of the pellets into finished articles. However, when the intention is to produce finished articles from, but not limited to, butylene and/or propylene polymer with very low melt viscosity, i.e. very high melt index, it can be difficult to carry out this degradation treatment effectively. When the degradation treatment is carried out in a pelletizing extruder, the polymer leaving the extruder becomes so fluid and so soft that it is difficult or even impossible to cut into pellet form. Moreover, the pellets consisting of these polymers of very high melt index can be sticky and can tend to agglomerate, making handling very difficult.
Attempts to pelletize ultra low melt viscosity polyolefins cracked by the conventional processes result in an excess amount of non-uniform or malformed pellets which may be described by terms such as tailed pellets, long-string pellets, pellet marriages, elbows, dog bones, and pellet trash. Malformed and non-uniform pellets are undesirable since they tend to bridge in pellet feed hoppers and to block pellet conveying systems. Further, significant amounts of malformed pellets alter the bulk density of the pellet stock which causes feeding problems in the extrusion line and which may result in voids in the final product. In addition to malformed pellets, trashouts occur frequently during production of ultra low melt viscosity polyolefins. Trashouts are extruder shutdowns resulting from polymer buildup on the rotating knives.
The user of the pellets, i.e. the converter, is generally someone other than the manufacturer of the polymers and of the pellets. When the converter employs this degradation treatment during the conversion of the pellets into finished articles, he must modify and adapt the extruders or other converting devices and the conditions of their use, in order to effectively process each type of pellet. In particular, he must equip the extruders or other converting equipment with a device for introducing and metering the free radical generator, while satisfying safety constraints due to the thermal instability of these materials. It has been observed that when this degradation treatment is carried out with a polymer which is not in the form of a powder but in the form of pellets, the dispersion of the free radical generator in the polymer may be relatively more difficult and the lack of homogeneity of the mixture may locally result in excessive degradation of the polymer.
The users of the pellets usually mix the pellets in an appropriate equipment at a selected temperature with additional ingredients such as additives, other polymers, and antioxidants to form a blend in its molten state before converting the mixture containing the pellets into finished articles. Thus, the viscosity of the pellets must be sufficiently low so that the blending or mixing operation can be successfully conducted without causing mechanical breakdown of the mixing stirrer because of the high shear resistance from the molten mixture.
It is therefore desirable to crack the polymer during the extrusion pelletization stage as much as possible to meet the melt viscosity specification required by the end users, while at the same time stay within the melt viscosity range which cracked polymer can easily be mechanically cut into non-sticky uniform pellets by the pelletizer.
It is also desirable to control the consistency of the degree of degradation in the extruder to have a narrow molecular weight distribution of the polymer in the pellets to prepare pellets of a quality which is as constant as possible. It is further desirable to control the constancy of the concentration of the live or intact, i.e. unreacted, free radical generators in the pellets. Such reproductability in the manufacture of the pellets advantageously reduces the need for the end users, the converters, to constantly change the settings of the converting equipment intended to convert the pellets into finished articles.
U.S. Pat. No. 4,451,589, assigned to Kimberly-Clark proposes a degradation process wherein greater than about 50% added peroxide remains available for further degradation after pelletizing. The peroxide can be added either prior to or during the extrusion process. No mention was made to multiple additions of peroxide during the extrusion process. The single point addition of large amount of peroxide as proposed by this reference often end up with lack of control of the consistency of the degree of degradation and thus a large percentage of recoverable and unrecoverable products which do not meet the blending and/or shipping specifications.
U.S. Pat. No. 4,897,452, assigned to BP Chemicals, proposes a degradation process involving adding to the polymer two free radical generators, G1 and G2, the half-life of G2 being at least 20 times longer than that of G1 at the pelletizing temperature. This process requires the use of G2 with relatively long half life. The process has the disadvantage of requiring high temperature and/or prolonged heating during the conversion of the pellets into finished articles by the converters or end users in order to completely decompose the G2 free radical generators.
Thus, a need has been demonstrated for a process of degrading a polyolefin, producing polymer pellets of a constant quality, including constancy in viscosity, in a reproducible manner, while minimizing the viscosity of the polymer to meet the viscosity specification required by the customer, at the same time being easily cut into non-sticky pellets without excessive free radical prodegradant, and when heated undergoes further degradation producing a ultra low melt viscosity polymer without excessive heating.